Formation of nitrogen oxides in furnaces

The onset temperature for the significant formation of nitrogen oxides (mainly thermal NOx) in gas boilers is around 1300–1400°C, and the formation rate increases exponentially as the temperature exceeds 1500°C.

The NOX temperature in coal furnaces is 1300–1500 degrees Celsius; it increases significantly when it exceeds 1500 degrees Celsius. In other words, the NOX generation temperature in gas furnaces is the same as that in coal furnaces, but gas furnaces cannot reach 1300–1500 degrees Celsius, right?

This post was last edited by qqgd on 2026-6-15 at 20:39. The nitrogen compounds in coal, which serve as fuel in coal boilers, generally exist in the form of organic compounds. During high-temperature combustion, especially when the temperature exceeds 1200°C, the amount of nitrogen oxides increases significantly. These nitrogen oxides are primarily formed as a result of the combustion of nitrogen-containing organic compounds in the fuel, rather than from nitrogen in the air used for combustion; this is determined by the thermodynamic properties of organic nitrogen compounds during combustion. In gas furnaces, the fuel consists of light hydrocarbons—primarily methane and ethane (with possibly a certain proportion of hydrogen). The content of organic nitrogen compounds is extremely low. During combustion, the temperature can exceed 1200°C, and may even reach 1500°C. However, due to the inherent thermodynamic stability of nitrogen molecules in the combustion air, the amount of nitrogen oxides formed during combustion remains very low (although this proportion increases significantly compared to when the temperature is 1200°C). Nevertheless, at such high temperatures, the proportion of organic nitrogen compounds present in the gas (which are present in very low amounts) that get converted into nitrogen oxides increases substantially. The temperature at which nitrogen in the air reacts with oxygen to form nitrogen oxides is extremely high, generally requiring temperatures above 2500°C; in other words, these compounds are formed prominently at the temperatures associated with spark discharges, that is, during thunderstorm weather with lightning. This is also the main reason why nitrate and nitrite ions can be detected in rainwater after thunderstorms. That’s how nitrogen oxides came about in the Earth’s primitive atmosphere. If high-temperature combustion can successfully combine nitrogen with oxygen to form nitrogen oxides, it becomes less difficult to produce nitric acid industrially, and the synthesis of nitrogen fertilizers (nitrogenous compounds, not synthetic ammonia) also becomes simpler.

In a reformed 4-in-1 furnace, the temperature in the furnace chamber is at most around 780 degrees Celsius; shouldn’t nitrogen oxides not be produced in such conditions?

You mentioned a furnace temperature of 780, but when a heating furnace is burning, the local flame temperature can exceed 2000. The principle behind low-nitrogen burners is to use multiple nozzles within a single burner in order to reduce the local temperature, as high local flame temperatures are a problem

Does having more distributed burners on a burner reduce NOX formation? Thank you

Certainly, the more dispersed it is, with no hot spots in certain areas, nitrogen oxides will decrease

I’ve learned something from this! ! Thank you

The significant formation of nitrogen oxides (mainly thermal NOx) in gas boilers begins at a temperature of around 1300–1400°C, with a threshold for a sharp increase typically at 1500°C.